All eyes are on National Party MPs, the office of the Speaker and Parliamentary Service now the hunt for the leaker has begun.

National MPs will all be required to sign an access waiver allowing their computer systems to be forensically searched - something the party's shadow leader of the House Gerry Brownlee said he had already been approved.

Not even Mr Mallard was free of the waiver - although he said it was already on the record that neither he nor his office received the travel expenses electronically, which was how they were leaked to media.

Mr Mallard said there was no escaping cyber experts.

"The inquiry will look at who forwarded [the information] to whom, and also who else had access to the data which was very specific data at a very specific point in time," Mr Mallard said.

"The general manager of the Parliamentary Service has used his authority to give full access to all of the core Parliamentary Service computers for that purpose, so there is not a question of having to ask people's permission."

Gerry Brownlee Photo: VNP / Phil Smith

Mr Brownlee welcomed the investigation and said National had no issues with any of the inquiry's waiver requirements because they were all "hot under the collar" about the leak.

"Anything that goes into a server stays there no matter what you do with it."

ACT leader David Seymour agreed with Mr Brownlee and said if there was any chance of a leak from Parliamentary Service then it needed to be addressed.

"Parliamentary Service is responsible for my emails - there's confidential information about my constituents in the Epsom electorate.

"The idea that that could be compromised goes to the heart of our democracy and the low corruption expectations all New Zealanders rightly have."